Only cardinals under the age of 80 when the “sede vacante,” or the period between the death or lawful resignation of one pope and the election of his successor, begins are eligible to enter the conclave and vote for the next pope.

The voting by cardinals to elect the next pope takes place behind the locked doors of the Sistine Chapel, following a highly detailed procedure last revised by Pope Benedict XVI. Under the rules, secret ballots can be cast once on the first day of the conclave, then normally twice during each subsequent morning and evening session. Except for periodic pauses, the voting continues until a new pontiff is elected.

In theory, any baptized male Catholic can be elected pope, but current church law says he must become a bishop before taking office; since the 15th century, the electors always have chosen a fellow cardinal.

If a conclave has not elected a pope after 13 days, the cardinals pause for a day of prayer, reflection and dialogue, then move to a runoff election between the two cardinals who obtained the most votes on the previous ballot. The two leading cardinals do not vote in the runoff ballots, though they remain in the Sistine Chapel.

When a pope is elected, the ballots are burned immediately. By tradition, the ballots are burned dry—or with chemical additives—to produce white smoke when a pope has been elected; they are burned with damp straw or other chemicals to produce black smoke when the voting has been inconclusive.